The stamping process is well-known for mass production. However, the traditional stamping process requires massive time to design the mold and optimize the process conditions, and the miniaturization of the flow channels dimensions makes the process more complex. The wrinkle and rupture are the main defects in the metal bipolar plates stamping process. Moreover, undesirable dimensional variability in the channel depth of a metal bipolar plate presents performance issues among other issues.
The dimensions of the flow channel and the conditions of the stamping process play important roles on the formability of stamping process. Research on dimension design has been developed widely. However, attention has mostly focused on the effect of channel dimensions and the efficiency of flow channel while less attention has been focused on the effects when forming a flow channel.
As is generally known, stamped components are made by forming, trimming, blanking or piercing metal—in sheet or coil form between two halves (upper and lower) of a stamping press tool called a die assembly. The upper member or members are attached to slide or slides of the press and the lower member is clamped or bolted to the bed or bolster. The die is designed to create the shape and size of a component. The two halves of the die are brought together in the press. Both force (load) and accuracy are required to achieve the repeatability and tolerance requirements.
The die assembly used in a stamping press is a special, one-of-a-kind precision tool that cuts and forms sheet metal 46 into a desired shape or profile—such as a bipolar plate having flow channels and metal beads. The die's cutting and forming sections typically are made from special types of hardenable steel called tool steel. Dies also can contain cutting and forming sections made from carbide or various other hard, wear-resistant materials.
Most stamping dies are constructed of several basic components which may include die plates, shoes, die sets, guide pins, bushings, heel blocks, heel plates, screws, dowels, and keys. Dies also need stripper, pressure, and drawing pads, as well as the devices used to secure them; spools, shoulder bolts, keepers, and retainers, as well as gas, coil, or urethane springs.
Die plates, shoes, and die sets are steel or aluminum plates that correspond to the size of the die. The die shoes serve as the foundation for mounting the working die components. Most die shoes are made from steel. Aluminum also is a popular die shoe material. Aluminum is one-third the weight of steel, it can be machined very quickly, and special alloys can be added to it to give it greater compressive strength than low-carbon steel. Aluminum also is a great metal for shock adsorption, which makes it a good choice for blanking dies. The upper and lower die shoes are assembled together with guide pins in order to create the die set or die assembly. Guide pins, sometimes referred to as guide posts or pillars, function together with guide bushings to align both the upper and lower die shoes precisely in a stamping press.
Referring again to FIG. 1, the traditional bipolar plate is shown. The bipolar plate divides the unit cells in the fuel cell stack, and at the same time, serves as a current path (a path for transferring generated electricity) between the unit cells. The bipolar plate 110 may generally have a rectangular shape. The bipolar plate 110 has a reaction region 130 which has flow fields 131 for air, hydrogen, and coolant. Opposite end portions of the reaction region 130 have inlet manifold holes 132 and exit manifold holes 134 through which air, hydrogen, and coolant enter and exit, respectively.
The flow fields or flow channels 112 formed in the bipolar plate 110 serve as a path for transferring reactant gases to the GDL, a path for the pass of coolant, and a path for discharging water, which is produced by the electrochemical reaction and is discharged through the GDL, to the outside. However, it is difficult for the metallic bipolar plate 110 manufactured by a stamping press to achieve the optimum complex shape with very tight tolerances.
As indicated, bipolar plates are manufactured by forming relief/patterns (flow channels and beads) in a metal plate via a stamping press. Two bipolar plates are then coupled to each other. Accordingly, coolant flows in a channel space defined by contact of the bipolar plates, and Gas Diffusion Layers (GDL's) are disposed at both sides of the bipolar plates so that hydrogen and oxygen flow in respective channel spaces defined between the GDLs and the bipolar plates so as to transfer reactant gases. However, due to the significant forces imposed on the metal plate during the stamping process, the center region 52 of the die assembly (upper and/or lower die sets) will tend to cave in relative to the outer regions 50. This causes the load applied in the stamping process to be non-uniform thereby causing undesirably uneven depth within the flow channels and metal beads. As indicated earlier, efficient performance from a bipolar plate requires uniform channel depth as well as uniform bead depth.
With reference again to FIG. 1, the example bipolar plate 110 having flow channels 112 for use in a PEM fuel cell stack is shown. It is understood that the bipolar plate 110 may be made from a sheet of steel. It is understood that it is critical to maintain a substantially uniform flow channels 112 and metal bead seal 114 depth in order to provide a robust and efficiently operating structure. The sheet metal used in bipolar plates are approximate in the range of 0.07 to 0.12 thick. Moreover, flow channel 112 depths in a bipolar plate may approximately be in the range of 0.2 to 0.8. While the metal beads should preferably have a depth in the approximate range of 0.4 to 1.2. The depth of the flow channels should be substantially uniform and the depth of the metal beads should be substantially uniform in order to obtain efficient performance from the bipolar plate/fuel cell. In order to achieve uniform deformation in the channels and bead seals across the width and length of the bipolar plate (via a microstamping process), the die assembly should remain rigid in a stamping press and apply an average forming pressure evenly across the sheet metal. That is, the die face needs to be applied evenly across the sheet metal in order to achieve uniform channel depth and uniform bead depth.
Referring now to FIG. 2, a traditional die assembly 116 is shown for use in a stamping press (not shown). The traditional die assembly 116 includes a die plate 118 which has curves and recesses (not shown) formed in the die plate 118 for shaping at least a portion of sheet metal that is inserted into a stamping press machine. The die plate 118 may be mounted on a flat die stiffener 120 as shown. The flat die stiffener 120 is then affixed to a die shoe 122 and the die shoe 122 is mounted on a press base 124 as shown. However, due to significant loads in bipolar plate forming that are applied to the traditional die assembly shown, the inner region 126 of the die plate 118 and the die stiffener 122 generally begins to cave in relative to the outer regions 128 of the die plate 118 and the die stiffener 120. Therefore, any channels or formations that need to be formed in a piece of sheet metal may not reach their desired depth due to the deformation in the center region of the traditional die assembly.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. Accordingly, there is a need for an improved die assembly which forms stamped components with much greater dimensional accuracy when evenly applying a significant load and deformation across the die and sheet metal.